Hi John and welcome to GoY not sure where you live in the UK but if you are in the area that has had the prolonged drought the bush is shedding leaves to conserve water.

The most likely suspect is the gooseberry sawfly, a little green caterpillar which can strip off all the leaves in a couple of days. It has at least three generations and is active from May to September. Did you notice any of the leaves with holes in them before this occurred?
Don't worry, all is not lost, as the plant may well grow new leaves to build up its strength before winter. The worst that will have happened is that it will have weakened the new young plant.
Next year keep an eye out for the pest from April right through the season, and squash/crush any of the eggs and larvae you see. You can spray, but then you don't want gooseberries polluted with chemicals. You could also try covering the bushes with the very fine netting now available.